A biological visual system is a capacity limited system in that it can only process a relatively small number of objects at any given time. This is true, despite the fact that there are many objects that may be visible at any given time. From the array of objects visible to a person, that person's visual system will only attend to, or process, one (or a few) objects at any given time. In addition, people can attend to an object while looking at it, which is overt attention, and people can attend to an object without looking at it in their peripheral vision, which is covert attention.
Understanding what attracts visual attention is a topic of research in psychology, neuroscience and computer science. This research has generated numerous studies directed toward understanding the behavior of human visual attention, as well as many computational models of visual attention. These computational models (sometimes called visual attention models, eye-gaze prediction models, attention models, or saliency models) attempt to simulate where, given visual stimuli (for example, a picture or a scene), a person will allocate his visual attention.